Thurston becomes Shields' first summer signing
The 24-year-old, who can also play at right-back, has signed subject to receiving international clearance.
Thurston has spent the last two years playing in the Southern Hemisphere, most recently for Canterbury United in New Zealand's top flight.
He spent eight years in Manchester United's academy before spells with Preston North End, Hyde United and FC United of Manchester.
Thurston - originally from the North West - came to the attention of Shields joint managers Lee Picton and Graham Fenton after his return to the country this summer. He has now put pen to paper and is excited about the future at Mariners Park.
He said: "I'm buzzing to be here. It took a little while to get it over the line but now it's done, I'm delighted and can't wait to get going.
"The club has big ambitions and I'm excited to be a part of it."
Thurston prefers a central midfield role and is looking forward to returning to the style of football he knows best after his spells abroad.
He added: "I learned a lot from being away in a different culture and environment.
"I met a lot of good people and loved every minute of it, but I'm delighted to be back and being involved in more competitive football. It's a lot more physical over here.”
"I've played in this league before and it's tough because there are no easy games and it's always competitive.
"Promotion is our aim next season and Lee and Graham have excited me about the plan to move up through the leagues, so I can't wait to get started."
Picton added: "We're delighted to be bringing someone of Adam's calibre to the club.
"His footballing journey has brought him through clubs of the stature of Manchester United and Preston, and he has played in the top flight of New Zealand football for the last couple of seasons with Canterbury.
"Adam is a fantastic age and has impressed us massively whenever we have watched him play or met him in person as a human being.”